Golf ball

ABSTRACT

An object of the present invention is to provide a golf ball having excellent shot feeling on driver shots. The present invention provides a golf ball comprising a core and at least one cover covering the core, wherein the core is formed from a core rubber composition containing (a) a base rubber containing a polybutadiene, (b) an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or a metal salt thereof as a co-crosslinking agent, (c) a crosslinking initiator, and (d) a terpene-based resin.

FIELD OF THE INVENTION

The present invention relates to a golf ball, more specifically relatesto an improvement in a core of a golf ball.

DESCRIPTION OF THE RELATED ART

As a material for forming a core of a golf ball, a rubber compositioncontaining a base rubber, a co-crosslinking agent, and a crosslinkinginitiator is widely used, in light of its good resilience.

For example, JP 2016-019620 A discloses a golf ball comprising aspherical core and at least one cover covering the spherical core,wherein the spherical core is formed from a rubber compositioncontaining (a) a base rubber, (b) an α,β-unsaturated carboxylic acidhaving 3 to 8 carbon atoms and/or a metal salt thereof as aco-crosslinking agent, (c) a crosslinking initiator and (d) a petroleumresin, and further containing (e) a metal compound in case that (b) theco-crosslinking agent consists of the α,β-unsaturated carboxylic acidhaving 3 to 8 carbon atoms.

In addition, JP S63-54181 A discloses a golf ball comprising a core,wherein the core is composed of a viscoelastic material having a reboundresilience (based on JIS K 630) of 40% or less and an interior losscoefficient tan δ showing a maximum value of 0.3 or more at a range offrom 0° C. to 40° C., and a constituent material of the core is a rubbercomposition obtained by blending an ordinary additive such as avulcanization agent, a vulcanization accelerator, a lubricant or aplasticizer, and further a large amount of a filler, a softening agentor a resin such as a petroleum resin or a terpene resin into butylrubber, ethylene-propylene-diene terpolymer, high styrene rubber,ethylene-propylene rubber, styrene-butadiene rubber,acrylonitrile-butadiene rubber or norbornene polymer.

JP 2008-126062 A discloses a multipiece golf ball comprising at leastthree core layers including a center, an inner core layer and an outercore layer, at least one cover, and at least one water vapor barrierlayer disposed between the outer core layer and the at least one coverto realize greater water vapor permeability than the cover, wherein ahardness gradient is provided to the center, the inner core layer andthe outer core layer, and discloses a terpene resin, a terpene resinester or the like may be added in the water vapor barrier layer.

JP 2006-320725 A discloses a golf ball comprising a core, anintermediate layer and a cover, wherein the intermediate layer isproduced from a composition containing an ionomer resin and a rosinmaterial having a softening point of at least about 50° C.

JP 2005-185836 A discloses a golf ball comprising a core and a cover,wherein at least one of the core and the cover contains a plasticizedpolyurethane composition containing at least one polyurethane and atleast one plasticizer.

JP 2001-137386 A discloses a golf ball comprising a core and a cover,wherein the cover has a loss tangent (tan δ) value in a range of from0.15 to 0.70 at −10° C. in a temperature distribution curve for dynamicviscoelasticity measured in a tensile mode under conditions oftemperature rising rate: 4° C./min, frequency: 10 Hz, initial strain:1.0 mm, and the cover composition contains a material consisting of anionomer resin, or a mixed material primarily containing an ionomer resinand further containing one or at least two elastomers having a rubberelement, and a terpene resin and/or a rosin ester resin used as atackifier.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a golf ball havingexcellent shot feeling on driver shots by improving a core of the golfball.

The present invention provides a golf ball comprising a core and atleast one cover covering the core, wherein the core is formed from acore rubber composition containing (a) a base rubber containing apolybutadiene. (b) an α,β-unsaturated carboxylic acid having 3 to 8carbon atoms and/or a metal salt thereof as a co-crosslinking agent, (c)a crosslinking initiator, and (d) a terpene-based resin.

The core formed from the rubber composition containing (d) theterpene-based resin has a lowered hardness on the core surface portion.As a result, a golf ball having excellent shot feeling on driver shotsis obtained.

According to the present invention, a golf ball having excellent shotfeeling on driver shots can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

The FIGURE is a partially cutaway cross-sectional view of a golf ballaccording to one embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention provides a golf ball comprising a core and atleast one cover covering the core, wherein the core is formed from acore rubber composition containing (a) a base rubber containing apolybutadiene. (b) an α,β-unsaturated carboxylic acid having 3 to 8carbon atoms and/or a metal salt thereof as a co-crosslinking agent, (c)a crosslinking initiator, and (d) a terpene-based resin. In thefollowings, the materials used for the present invention will bedescribed.

[(a) Base Rubber]

(a) The base rubber used for the core rubber composition according tothe present invention preferably contains a polybutadiene. As thepolybutadiene, a high-cis polybutadiene having a cis-1,4 bond in anamount of 90 mass % or more (hereinafter sometimes simply referred to“high-cis polybutadiene”) is preferable.

The high-cis polybutadiene preferably has a 1,2-vinyl bond in an amountof 2.0 mass % or less, more preferably 1.7 mass % or less, and even morepreferably 1.5 mass % or less. This is because if the amount of the1,2-vinyl bond is excessively high, the resilience may be lowered.

The high-cis polybutadiene is preferably one synthesized using arare-earth element catalyst. When a neodymium catalyst employing aneodymium compound which is a lanthanum series rare-earth elementcompound, is used, a polybutadiene rubber having a high amount of thecis-1,4 bond and a low amount of the 1,2-vinyl bond is obtained with anexcellent polymerization activity, and thus such polybutadiene rubber isparticularly preferred.

The polybutadiene preferably has a Mooney viscosity (ML₁₊₄ (100° C.)) of30 or more, more preferably 32 or more, and even more preferably 35 ormore, and preferably has a Mooney viscosity (ML₁₊₄ (100° C.)) of 140 orless, more preferably 120 or less, even more preferably 100 or less, andmost preferably 80 or less. It is noted that the Mooney viscosity (ML₁₊₄(100° C.)) in the present invention is a value measured according to JISK6300 using an L rotor under the conditions of preheating time: 1minute, rotor rotation time: 4 minutes, and temperature: 100° C.

The polybutadiene preferably has a molecular weight distribution Mw/Mn(Mw: weight average molecular weight, Mn: number average molecularweight) of 2.0 or more, more preferably 2.2 or more, even morepreferably 2.4 or more, and most preferably 2.6 or more, and preferablyhas a molecular weight distribution Mw/Mn of 6.0 or less, morepreferably 5.0 or less, even more preferably 4.0 or less, and mostpreferably 3.4 or less. This is because if the molecular weightdistribution (Mw/Mn) of the polybutadiene is excessively low, theprocessability may deteriorate, and if the molecular weight distribution(Mw/Mn) of the polybutadiene is excessively high, the resilience may belowered. It is noted that the molecular weight distribution is measuredby gel permeation chromatography (“HLC-8120GPC” available from TosohCorporation) using a differential refractometer as a detector under theconditions of column: GMHHXL (available from Tosoh Corporation), columntemperature: 40° C., and mobile phase: tetrahydrofuran, and calculatedby converting based on polystyrene standard.

From the viewpoint of obtaining a core having higher resilience, theamount of the polybutadiene in the base rubber is preferably 60 mass %or more, more preferably 80 mass % or more, and even more preferably 90mass % or more. It is also preferred that (a) the base rubber consistsof the polybutadiene.

(a) The base rubber may further contain another rubber in addition tothe polybutadiene rubber. Examples of another rubber include naturalrubber, polyisoprene rubber, styrene polybutadiene rubber, andethylene-propylene-diene rubber (EPDM). These rubbers may be used solelyor in combination of at least two of them.

[(b) Co-Crosslinking Agent]

(b) The α,β-unsaturated carboxylic acid having 3 to 8 carbon atomsand/or the metal salt thereof used in the core rubber composition isblended as a co-crosslinking agent in the rubber composition, and has anaction of crosslinking a rubber molecule by graft polymerization to abase rubber molecular chain.

Examples of the α,β-unsaturated carboxylic acid having 3 to 8 carbonatoms include acrylic acid, methacrylic acid, fumaric acid, maleic acid,and crotonic acid.

Examples of the metal constituting the metal salt of the α,β-unsaturatedcarboxylic acid having 3 to 8 carbon atoms include a monovalent metalion such as sodium, potassium, and lithium; a divalent metal ion such asmagnesium, calcium, zinc, barium, and cadmium; a trivalent metal ionsuch as aluminum; and other metal ions such as tin, and zirconium. Themetal component may be used solely or as a mixture of at least two ofthem. Among them, as the metal component, the divalent metal such asmagnesium, calcium, zinc, barium, and cadmium is preferred. This isbecause use of the divalent metal salt of the α,β-unsaturated carboxylicacid having 3 to 8 carbon atoms easily generates a metal crosslinkingbetween the rubber molecules. Especially, as the divalent metal salt,zinc acrylate is preferred, because zinc acrylate enhances theresilience of the resultant golf ball. The α,β-unsaturated carboxylicacid having 3 to 8 carbon atoms and/or the metal salt thereof may beused solely or in combination of at least two of them.

The amount of (b) the α,β-unsaturated carboxylic acid having 3 to 8carbon atoms and/or the metal salt thereof is preferably 15 parts bymass or more, more preferably 20 parts by mass or more, and even morepreferably 25 parts by mass or more, and is preferably 50 parts by massor less, more preferably 45 parts by mass or less, and even morepreferably 35 parts by mass or less, with respect to 100 parts by massof (a) the base rubber. If the amount of (b) the α,β-unsaturatedcarboxylic acid having 3 to 8 carbon atoms and/or the metal salt thereofis less than 15 parts by mass, the amount of (c) the crosslinkinginitiator which will be explained below must be increased in order toobtain an appropriate hardness of the core formed from the core rubbercomposition, which tends to lower the resilience of the obtained golfball. On the other hand, if the amount of the α,β-unsaturated carboxylicacid having 3 to 8 carbon atoms and/or the metal salt thereof is morethan 50 parts by mass, the core formed from the core rubber compositionbecomes so hard that the shot feeling of the obtained golf ball may belowered.

[(c) Crosslinking Initiator]

(c) The crosslinking initiator used in the core rubber composition isblended in order to crosslink (a) the base rubber component. As (c) thecrosslinking initiator, an organic peroxide is preferred. Specificexamples of the organic peroxide include an organic peroxide such asdicumyl peroxide, 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane,2,5-dimethyl-2,5-di(t-butylperoxy)hexane, and di-t-butylperoxide. Theseorganic peroxides may be used solely or in combination of at least twoof them. Among them, dicumyl peroxide is preferably used.

The amount of (c) the crosslinking initiator is preferably 0.2 part bymass or more, more preferably 0.5 part by mass or more, and even morepreferably 0.7 part by mass or more, and is preferably 5.0 parts by massor less, more preferably 2.5 parts by mass or less, and even morepreferably 2.0 parts by mass or less, with respect to 100 parts by massof (a) the base rubber. If the amount of the crosslinking initiator isless than 0.2 part by mass, the core formed from the core rubbercomposition becomes so soft that the resilience of the obtained golfball may be lowered. If the amount of (c) the crosslinking initiator ismore than 5.0 parts by mass, the amount of (b) the co-crosslinking agentwhich has been explained above must be decreased in order to obtain anappropriate hardness of the core formed from the core rubbercomposition, which may lower the resilience of the obtained golf ball orworsen the durability of the obtained golf ball.

[(d) Terpene-Based Resin]

The terpene-based resin used in the present invention is notparticularly limited, as long as it is a polymer having a terpenecompound as a constituent component. The terpene-based resin ispreferably, for example, at least one member selected from the groupconsisting of a terpene polymer, a terpene-phenol copolymer, aterpene-styrene copolymer, a terpene-phenol-styrene copolymer, ahydrogenated terpene-phenol copolymer, a hydrogenated terpene-styrenecopolymer, and a hydrogenated terpene-phenol-styrene copolymer.

The terpene polymer is a homopolymer obtained by polymerizing a terpenecompound. The terpene compound includes a hydrocarbon represented by acomposition of (C₅H₈)_(n) and an oxygen-containing derivative thereof,and is a compound having a terpene such as monoterpene (C₁₀H₁₅),sesquiterpene (C₁₅H₂₄) or diterpene (C₂₀H₃₂) as a basic skeleton.Examples of the terpene compound include α-pinene, β-pinene, dipentene,limonene, myrcene, alloocimene, ocimene, α-phellandrene, α-terpinene,γ-terpinene, terpinolene, 1,8-cineole, 1,4-cineole, α-terpineol,β-terpineol, and γ-terpineol. The terpene compound may be used solely orused as a mixture of two or more of them.

The terpene polymer is obtained, for example, by polymerizing the aboveterpene compound. Examples of the terpene polymer include α-pinenepolymer, β-pinene polymer, limonene polymer, dipentene polymer, andf-pinene/limonene polymer.

The terpene-phenol copolymer (hereinafter sometimes simply referred toas “terpene phenolic resin”) is, for example, a copolymer of the aboveterpene compound and a phenol-based compound. Examples of thephenol-based compound include phenol, cresol, xylenol, catechol,resorcin, hydroquinone, and bisphenol A. As the terpene-phenolcopolymer, the copolymer of the above terpene compound and phenol ispreferable.

The acid value of the terpene-phenol copolymer is preferably 10 mgKOH/gor more, more preferably 35 mgKOH/g or more, and even more preferably 60mgKOHg or more. In addition, the acid value of the terpene-phenolcopolymer is preferably 300 mgKOH/g or less, more preferably 250 mgKOH/gor less, even more preferably 200 mgKOH/g or less, particularlypreferably 150 mgKOH/g or less, and most preferably 90 mgKOH/g or less.It is noted that, in the present invention, the acid value of theterpene-phenol copolymer is an amount in milligram of potassiumhydroxide required for neutralizing the acid included in one gram of theterpene-phenol copolymer, and is a value measured by a potentiometrictitration method (JIS K 0070: 1992).

The hydroxy value of the terpene-phenol copolymer is preferably 30mgKOH/g or more, more preferably 50 mgKOH/g or more. The hydroxy valueof the terpene-phenol copolymer is preferably 150 mgKOH/g or less, morepreferably 100 mgKOH/g or less. It is noted that, in the presentspecification, the hydroxy value is an amount in milligram of potassiumhydroxide required for neutralizing acetic acid bonding to hydroxylgroup when one gram of the resin is acetylated, and is a value measuredby a potentiometric titration method (JIS K 0070: 1992).

The terpene-styrene copolymer is, for example, a copolymer of the aboveterpene compound and a styrene-based compound. Examples of thestyrene-based compound include styrene, and α-methylstyrene. As theterpene-styrene copolymer, the copolymer of the above terpene compoundand α-methylstyrene is preferable.

The terpene-phenol-styrene copolymer is, for example, a copolymer of theabove terpene compound, the above phenolic compound and the abovestyrene-based compound. As the terpene-phenol-styrene copolymer, thecopolymer of the above terpene compound, phenol and α-methylstyrene ispreferable.

The hydrogenated terpene-phenol copolymer is one obtained byhydrogenating the above terpene-phenol copolymer. The hydrogenatedterpene-styrene copolymer is one obtained by hydrogenating the aboveterpene-styrene copolymer. The hydrogenated terpene-phenol-styrenecopolymer is one obtained by hydrogenating the aboveterpene-phenol-styrene copolymer.

As (d) the terpene-based resin, at least one member selected from thegroup consisting of compounds having a structure represented by thefollowing formulae (1) to (4) is preferable.

[In the formulae (1) to (4), R¹ and R² each independently represent adivalent residue of a phenol-based compound and/or a styrene-basedcompound, m¹ to m⁴ each independently represent a natural number of 1 to30, and n¹ to n² each independently represent a natural number of 1 to20.]

The compounds having the structure represented by the above formulae (1)to (4) each have a structure derived from pinene in the molecule.

The compound having the structure represented by the formula (1) has arepeating unit consisting of a structural moiety derived from α-pineneand R¹ bonding to the structural moiety derived from α-pinene. R¹ ispreferably a divalent residue where two hydrogen atoms are removed frombenzene ring of a phenolic compound and/or a styrene-based compound.Examples of the compound having the structure represented by the formula(1) include a copolymer of α-pinene and a phenol-based compound and/or astyrene-based compound.

Examples of the phenol-based compound include phenol, cresol, xylenol,catechol, resorcin, hydroquinone, and bisphenol A. Examples of thestyrene-based compound include styrene, and α-methylstyrene.

In the formula (1), m¹ represents the degree of polymerization of thestructural unit derived from α-pinene, and is preferably a naturalnumber of 1 to 30. m¹ is preferably 1 or more, more preferably 2 ormore, and even more preferably 3 or more, and is preferably 30 or less,more preferably 25 or less, and even more preferably 20 or less.

In the formula (1), n¹ represents the degree of polymerization of therepeating unit consisting of the structural moiety derived from α-pineneand R¹ bonding to the structural moiety derived from α-pinene, and ispreferably a natural number of 1 to 20. n¹ is preferably 1 or more, morepreferably 2 or more, and even more preferably 3 or more, and ispreferably 20 or less, more preferably 18 or less, and even morepreferably 15 or less.

The compound having the structure represented by the formula (2) has arepeating unit consisting of a structural moiety derived from β-pineneand R² bonding to the structural moiety in the molecule. Examples of thecompound having the structure represented by the formula (2) include acopolymer of β-pinene and a phenol-based compound and/or a styrene-basedcompound. R² is a divalent residue where two hydrogen atoms are removedfrom benzene ring of a phenol-based compound and/or a styrene-basedcompound.

Examples of the phenol-based compound include phenol, cresol, xylenol,catechol, resorcin, hydroquinone, and bisphenol A. Examples of thestyrene-based compound include styrene, and α-methylstyrene.

In the formula (2), m² represents the degree of polymerization of thestructural unit derived from R-pinene, and is preferably a naturalnumber of 1 to 30. m² is preferably 1 or more, more preferably 2 ormore, and even more preferably 3 or more, and is preferably 30 or less,more preferably 25 or less, and even more preferably 20 or less.

In the formula (2), n² represents the degree of polymerization of therepeating unit consisting of a structural moiety derived from G-pineneand R² bonding to the structural moiety, and is preferably a naturalnumber of 1 to 20. n² is preferably 1 or more, more preferably 2 ormore, and even more preferably 3 or more, and is preferably 20 or less,more preferably 18 or less, and even more preferably 15 or less.

The compound having the structure represented by the formula (3) is apolymer having a structural unit derived from α-pinene, and is morepreferably a polymer consisting of the structural unit derived fromα-pinene.

In the formula (3), m³ represents the degree of polymerization of thestructural unit derived from α-pinene, and is preferably a naturalnumber of 1 to 30. m³ is preferably 1 or more, more preferably 2 ormore, and even more preferably 3 or more, and is preferably 30 or less,more preferably 25 or less, and even more preferably 20 or less.

The compound having the structure represented by the formula (4) is aβ-pinene polymer having a structural unit derived from β-pinene in themolecule, and is more preferably a polymer consisting of the structuralunit derived from β-pinene.

In the formula (4), m⁴ represents the degree of polymerization of thestructural unit derived from β-pinene, and is preferably a naturalnumber of 1 to 30. m⁴ is preferably 1 or more, more preferably 2 ormore, and even more preferably 3 or more, and is preferably 30 or less,more preferably 25 or less, and even more preferably 20 or less.

Particularly preferable examples of (d) the terpene-based resin includeα-pinene-phenol copolymer, α-pinene-α-methylstyrene copolymer,α-pinene-α-methylstyrene-phenol copolymer, β-pinene-phenol copolymer,β-pinene-α-methylstyrene copolymer, and β-pinene-α-methylstyrene-phenolcopolymer. As (d) the terpene-based resin, these copolymers may be usedsolely, or two or more of them may be used in combination.

The softening point of (d) the terpene-based resin is preferably 60° C.or more, more preferably 80° C. or more, and even more preferably 100°C. or more, and is preferably 150° C. or less, more preferably 130° C.or less, and even more preferably 120° C. or less. This is because useof (d) the terpene-based resin having the softening point falling withinthe above range improves the dispersibility of the resin during kneadingthe rubber. It is noted that the softening point of (d) theterpene-based resin is measured with a ring and ball type softeningpoint measuring apparatus according to JIS K 6220-1: 2001, and is atemperature at which the ball drops.

As (d) the terpene-based resin, a commercial product can be used.Examples of the commercial product include Sylvares TP2019 andSylvatraxx 6720 available from Kraton Corporation; and YS RESIN PX 1150Navailable from Yasuhara Chemical Co. Ltd.

The amount of (d) the terpene-based resin is preferably 3 parts by massor more, more preferably 4 parts by mass or more, and even morepreferably 5 parts by mass or more, and is preferably 20 parts by massor less, more preferably 18 parts by mass or less, and even morepreferably 15 parts by mass or less, with respect to 100 parts by massof (a) the base rubber. If the amount of the component (d) is less than3 parts by mass, the effect of adding the component (d) is small, andthus the improvement effect on the shot feeling on driver shots may notbe obtained. On the other hand, if the amount of the component (d) ismore than 20 parts by mass, the obtained core becomes excessively softas a whole, and thus the resilience may be lowered.

The blending ratio (the component (b)/the component (d)) of thecomponent (b) to the component (d) is preferably 2.0 or more, morepreferably 2.5 or more, and even more preferably 2.8 or more, and ispreferably 15.0 or less, more preferably 12.0 or less, even morepreferably 10.0 or less, and most preferably 8.0 or less in a massratio. If the blending ratio (the component (b)/the component (d)) ofthe component (b) to the component (d) falls within the above range, theobtained golf ball has better shot feeling on driver shots.

[(e) Organic Sulfur Compound]

The core rubber composition preferably further contains (e) an organicsulfur compound. If (e) the organic sulfur compound is contained, theobtained core has higher resilience.

As (e) the organic sulfur compound, at least one compound selected fromthe group consisting of thiols (thiophenols and thionaphthols),polysulfides, thiurams, thiocarboxylic acids, dithiocarboxylic acids,sulfenamides, dithiocarbamates and thiazoles is preferable.

Examples of the thiols include thiophenols and thionaphthols. Examplesof the thiophenols include thiophenol; thiophenols substituted with afluoro group, such as 4-fluorothiophenol, 2,4-difluorothiophenol,2,5-difluorothiophenol, 2,6-difluorothiophenol,2,4,5-trifluorothiophenol, and 2,4,5,6-tetrafluorothiophenol,pentafluorothiophenol; thiophenols substituted with a chloro group, suchas 2-chlorothiophenol, 4-chlorothiophenol, 2,4-dichlorothiophenol,2,5-dichlorothiophenol, 2,6-dichlorothiophenol,2,4,5-trichlorothiophenol, 2,4,5,6-tetrachlorothiophenol, andpentachlorothiophenol; thiophenols substituted with a bromo group, suchas 4-bromothiophenol, 2,4-dibromothiophenol, 2,5-dibromothiophenol,2,6-dibromothiophenol, 2,4,5-tribromothiophenol,2,4,5,6-tetrabromothiophenol, and pentabromothiophenol; thiophenolssubstituted with an iodo group, such as 4-iodothiophenol,2,4-diiodothiophenol, 2,5-diiodothiophenol, 2,6-diiodothiophenol,2,4,5-triiodothiophenol, and 2,4,5,6-tetraiodothiophenol,pentaiodothiophenol; and metal salts thereof. As the metal salt, a zincsalt is preferred.

Examples of the thionaphthols (naphthalenethiols) include2-thionaphthol, 1-thionaphthol, 1-chloro-2-thionaphthol,2-chloro-1-thionaphthol, 1-bromo-2-thionaphthol, 2-bromo-1-thionaphthol,1-fluoro-2-thionaphthol, 2-fluoro-1-thionaphthol,1-cyano-2-thionaphthol, 2-cyano-1-thionaphthol, 1-acetyl-2-thionaphthol,2-acetyl-1-thionaphthol, and a metal salt thereof. Preferable examplesinclude 2-thionaphthol, 1-thionaphthol, and a metal salt thereof. Themetal salt is preferably a divalent metal salt, more preferably a zincsalt. Specific examples of the metal salt include the zinc salt of1-thionaphthol and the zinc salt of 2-thionaphthol.

The polysulfides are organic sulfur compounds having a polysulfide bond,and examples thereof include disulfides, trisulfides, and tetrasulfides.As the polysulfides, diphenylpolysulfides are preferable.

Examples of the diphenylpolysulfides include diphenyldisulfide;diphenyldisulfides substituted with a halogen group, such asbis(4-fluorophenyl)disulfide, bis(2,5-difluorophenyl)disulfide,bis(2,6-difluorophenyl)disulfide, bis(2,4,5-trifluorophenyl)disulfide,bis(2,4,5,6-tetrafluorophenyl)disulfide,bis(pentafluorophenyl)disulfide, bis(4-chlorophenyl)disulfide,bis(2,5-dichlorophenyl)disulfide, bis(2,6-dichlorophenyl)disulfide,bis(2,4,5-trichlorophenyl)disulfide,bis(2,4,5,6-tetrachlorophenyl)disulfide,bis(pentachlorophenyl)disulfide, bis(4-bromophenyl)disulfide,bis(2,5-dibromophenyl)disulfide, bis(2,6-dibromophenyl)disulfide,bis(2,4,5-tribromophenyl)disulfide,bis(2,4,5,6-tetrabromophenyl)disulfide, bis(pentabromophenyl)disulfide,bis(4-iodophenyl)disulfide, bis(2,5-diiodophenyl)disulfide,bis(2,6-diiodophenyl)disulfide, bis(2,4,5-triiodophenyl)disulfide,bis(2,4,5,6-tetraiodophenyl)disulfide, andbis(pentaiodophenyl)disulfide; and diphenyldisulfides substituted withan alkyl group, such as bis(4-methylphenyl)disulfide,bis(2,4,5-trimethylphenyl)disulfide, bis(pentamethylphenyl)disulfide,bis(4-t-butylphenyl)disulfide, bis(2,4,5-tri-t-butylphenyl)disulfide,and bis(penta-t-butylphenyl)disulfide.

Examples of the thiurams include thiuram monosulfides such astetramethylthiuram monosulfide; thiuram disulfides such astetramethylthiuram disulfide, tetraethylthiuram disulfide, andtetrabutylthiuram disulfide; and thiuram tetrasulfides such asdipentamethylenethiuram tetrasulfide. Examples of the thiocarboxylicacids include a naphthalene thiocarboxylic acid. Examples of thedithiocarboxylic acids include a naphthalene dithiocarboxylic acid.Examples of the sulfenamides include N-cyclohexyl-2-benzothiazolesulfenamide. N-oxydiethylene-2-benzothiazole sulfenamide, andN-t-butyl-2-benzothiazole sulfenamide.

(e) The organic sulfur compound is preferably thiophenols and/or themetal salt thereof, thionaphthols and/or the metal salt thereof,diphenyldisulfides, and thiuramdisulfides, more preferably2,4-dichlorothiophenol, 2,6-difluorothiophenol, 2,6-dichlorothiophenol,2,6-dibromothiophenol, 2,6-diiodothiophenol, 2,4,5-trichlorothiophenol,pentachlorothiophenol, 1-thionaphthol, 2-thionaphthol,diphenyldisulfide, bis(2,6-difluorophenyl)disulfide,bis(2,6-dichlorophenyl)disulfide, bis(2,6-dibromophenyl)disulfide,bis(2,6-diiodophenyl)disulfide, and bis(pentabromophenyl) disulfide.

(e) The organic sulfur compound may be used solely or in combination ofat least two of them.

The amount of (e) the organic sulfur compound is preferably 0.05 part bymass or more, more preferably 0.1 part by mass or more, and even morepreferably 0.2 part by mass or more, and is preferably 5.0 parts by massor less, more preferably 3.0 parts by mass or less, and even morepreferably 2.0 parts by mass or less, with respect to 100 parts by massof (a) the base rubber. If the amount of (e) the organic sulfur compoundis less than 0.05 part by mass, the effect of adding (e) the organicsulfur compound may not be obtained, and thus the resilience of the golfball may not be improved. In addition, if the amount of (e) the organicsulfur compound is more than 5.0 parts by mass, the obtained golf ballmay have an excessively large compression deformation amount, and thusthe resilience thereof may be lowered.

[(f) Metal Compound]

In the case that the core rubber composition contains only theα,β-unsaturated carboxylic acid having 3 to 8 carbon atoms as theco-crosslinking agent, the core rubber composition preferably furthercontains (f) a metal compound. This is because neutralizing theα,β-unsaturated carboxylic acid having 3 to 8 carbon atoms with themetal compound in the core rubber composition provides substantially thesame effect as using the metal salt of the α,β-unsaturated carboxylicacid having 3 to 8 carbon atoms as the co-crosslinking agent. It isnoted that in case of using the α,β-unsaturated carboxylic acid having 3to 8 carbon atoms and the metal salt thereof in combination as theco-crosslinking agent, (f) the metal compound may be used as an optionalcomponent.

(f) The metal compound is not particularly limited as long as itneutralizes (b) the α,β-unsaturated carboxylic acid having 3 to 8 carbonatoms in the core rubber composition. Examples of (f) the metal compoundinclude a metal hydroxide such as magnesium hydroxide, zinc hydroxide,calcium hydroxide, sodium hydroxide, lithium hydroxide, potassiumhydroxide, and copper hydroxide; a metal oxide such as magnesium oxide,calcium oxide, zinc oxide, and copper oxide; and a metal carbonate suchas magnesium carbonate, zinc carbonate, calcium carbonate, sodiumcarbonate, lithium carbonate, and potassium carbonate. (f) The metalcompound is preferably a divalent metal compound, more preferably a zinccompound. This is because the divalent metal compound reacts with theα,β-unsaturated carboxylic acid having 3 to 8 carbon atoms, therebyforming a metal crosslinking. Further, use of the zinc compound providesa golf ball with higher resilience.

(f) The metal compound may be used solely or in combination of two ormore of them. In addition, the amount of (f) the metal compound may beappropriately adjusted in accordance with the desired neutralizationdegree of (b) the α,β-unsaturated carboxylic acid having 3 to 8 carbonatoms.

The core rubber composition used in the present invention may furthercontain additives such as a pigment, a filler for adjusting a weight orthe like, an antioxidant, a peptizing agent, a softening agent or thelike, where necessary.

The filler blended in the core rubber composition is used as a weightadjusting agent for mainly adjusting the weight of the golf ballobtained as a final product. The filler may be blended where necessary.Examples of the filler include an inorganic filler such as zinc oxide,barium sulfate, calcium carbonate, magnesium oxide, tungsten powder, andmolybdenum powder. Zinc oxide is particularly preferably used as thefiller. It is considered that zinc oxide functions as a vulcanizationactivator and increases the hardness of the entire core. The amount ofthe filler is preferably 0.5 part by mass or more, more preferably 1part by mass or more, and is preferably 30 parts by mass or less, morepreferably 25 parts by mass or less, and even more preferably 20 partsby mass or less, with respect to 100 parts by mass of (a) the baserubber. This is because if the amount of the filler is less than 0.5part by mass, it is difficult to adjust the weight, while if the amountof the filler exceeds 30 parts by mass, the weight ratio of the rubbercomponent is reduced and thus the resilience tends to be lowered.

The amount of the antioxidant is preferably 0.1 part by mass or more and1 part by mass or less with respect to 100 parts by mass of (a) the baserubber. In addition, the amount of the peptizing agent is preferably 0.1part by mass or more and 5 parts by mass or less with respect to 100parts by mass of (a) the base rubber.

[Core]

The core of the golf ball according to the present invention can beobtained by mixing and kneading the above-described core rubbercomposition, and molding the kneaded product in a mold. The moldingcondition is not particularly limited, and the molding is generallycarried out at 130° C. to 200° C. under a pressure of 2.9 MPa to 11.8MPa for 10 minutes to 60 minutes. For example, it is preferred that thecore rubber composition is heated at 130° C. to 200° C. for 10 to 60minutes, or alternatively the core rubber composition is subjected to atwo-step heating, i.e. the core rubber composition is heated at 130° C.to 150° C. for 20 to 40 minutes and then heated at 160° C. to 180° C.for 5 to 15 minutes.

In the core of the golf ball according to the present invention, thehardness (H75) at the point located at the distance of 75% of the coreradius from a center of the core is preferably 55 or more, morepreferably 60 or more, and even more preferably 65 or more, and ispreferably 80 or less, more preferably 75 or less, and even morepreferably 72 or less in Shore C hardness. If the hardness (H75) at thepoint located at the distance of 75% of the core radius is 55 or more inShore C hardness, the resilience of the core is better. In addition, ifthe hardness (H75) at the point located at the distance of 75% of thecore radius is 80 or less in Shore C hardness, the shot feeling of thecore is better.

The surface hardness (Hs) of the core of the golf ball according to thepresent invention is preferably 60 or more, more preferably 62 or more,and even more preferably 65 or more, and is preferably 83 or less, morepreferably 80 or less, and even more preferably 78 or less in Shore Chardness. If the surface hardness (Hs) of the core is 60 or more inShore C hardness, the resilience of the core is better. In addition, ifthe surface hardness (Hs) of the core is 83 or less in Shore C hardness,the shot feeling on driver shots is further enhanced.

The center hardness (Ho) of the core is preferably 30 or more, morepreferably 35 or more, and even more preferably 40 or more in Shore Chardness. If the center hardness (Ho) of the core is 30 or more in ShoreC hardness, the core does not become excessively soft, and thus theresilience is better. In addition, the center hardness (Ho) of the coreis preferably 70 or less, more preferably 68 or less, and even morepreferably 67 or less in Shore C hardness. If the center hardness (Ho)is 70 or less in Shore C hardness, the core does not become excessivelyhard, and thus the shot feeling is better.

In the core of the golf ball according to the present invention, thehardness difference (Hs−H75) between the hardness (H75) at the pointlocated at the distance of 75% of the core radius and the surfacehardness (Hs) of the core is preferably less than 10, more preferably 9or less, even more preferably 8 or less, and most preferably 7 or lessin Shore C hardness. If the hardness difference (Hs−H75) between thehardness (H75) at the point located at the distance of 75% of the coreradius and the surface hardness (Hs) of the core is less than 10 inShore C hardness, the part near the core surface becomes soft, and thusthe shot feeling on driver shots is remarkably enhanced. In addition,the lower limit of the hardness difference (Hs−H75) between the hardness(H75) at the point located at the distance of 75% of the core radius andthe surface hardness (Hs) of the core is not particularly limited, butthe hardness difference (Hs−H75) between the hardness (H75) at the pointlocated at the distance of 75% of the core radius and the surfacehardness (Hs) of the core is preferably 0 or more, more preferably 1 ormore, and even more preferably 2 or more in Shore C hardness.

The hardness difference (Hs−Ho) between the surface hardness (Hs) of thecore and the center hardness (Ho) of the core is preferably 5 or more,more preferably 6 or more, and even more preferably 8 or more, and ispreferably 35 or less, more preferably 30 or less, and even morepreferably 25 or less in Shore C hardness. If the hardness difference(Hs−Ho) between the surface hardness (Hs) of the core and the centerhardness (Ho) of the core is 5 or more in Shore C hardness, the obtainedgolf ball has better resilience. In addition, if the hardness difference(Hs−Ho) between the surface hardness (Hs) of the core and the centerhardness (Ho) of the core is 35 or less in Shore C hardness, theobtained golf ball has further enhanced shot feeling on driver shots.

The hardness difference (H75−Ho) between the hardness (H75) at the pointlocated at the distance of 75% of the core radius and the centerhardness (Ho) of the core is preferably 3 or more, more preferably 5 ormore, and even more preferably 7 or more, and is preferably 20 or less,more preferably 18 or less, and even more preferably 15 or less in ShoreC hardness. If the hardness difference (H75−Ho) between the hardness(H75) at the point located at the distance of 75% of the core radius andthe center hardness (Ho) of the core is 3 or more in Shore C hardness,the resilience of the core is better. In addition, if the hardnessdifference (H75−Ho) between the hardness (H75) at the point located atthe distance of 75% of the core radius and the center hardness (Ho) ofthe core is 20 or less in Shore C hardness, the durability of the coreis better.

The core of the golf ball according to the present invention preferablyhas a diameter of 34.8 mm or more, more preferably 36.8 mm or more, andeven more preferably 38.8 mm or more, and preferably has a diameter of42.2 mm or less, more preferably 41.8 mm or less, even more preferably41.2 mm or less, and most preferably 40.8 mm or less. If the core has adiameter of 34.8 mm or more, the thickness of the cover does not becometoo thick and thus the resilience becomes better. On the other hand, ifthe core has a diameter of 42.2 mm or less, the thickness of the coverdoes not become too thin and thus the cover functions better.

When the core has a diameter in a range from 34.8 mm to 42.2 mm, thecompression deformation amount of the core (shrinking amount of the corealong the compression direction) when applying a load from an initialload of 98 N to a final load of 1275 N is preferably 2.0 mm or more,more preferably 2.3 mm or more, and even more preferably 2.5 mm or more,and is preferably 5.0 mm or less, more preferably 4.5 mm or less, andeven more preferably 4.3 mm or less. If the compression deformationamount is 2.0 mm or more, the shot feeling becomes better and if thecompression deformation amount is 5.0 mm or less, the resilience becomesbetter.

[Cover]

The cover of the golf ball of the present invention is formed from acover composition containing a resin component. Examples of the resincomponent include an ionomer resin; a thermoplastic polyurethaneelastomer having a trade name of “Elastollan (registered trademark)”available from BASF Japan Ltd.; a thermoplastic polyamide elastomerhaving a trade name of “Pebax (registered trademark)” available fromArkema K. K.; a thermoplastic polyester elastomer having a trade name of“Hytrel (registered trademark)” available from Du Pont-Toray Co., Ltd.;and a thermoplastic styrene elastomer having a trade name of “Rabalon(registered trademark)” available from Mitsubishi Chemical Corporation.

Examples of the ionomer resin include a product prepared by neutralizingat least a part of carboxyl groups in a binary copolymer composed of anolefin and an α,β-unsaturated carboxylic acid having 3 to 8 carbon atomswith a metal ion; a product prepared by neutralizing at least a part ofcarboxyl groups in a ternary copolymer composed of an olefin, anα,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and anα,β-unsaturated carboxylic acid ester with a metal ion; and a mixture ofthose. The olefin is preferably an olefin having 2 to 8 carbon atoms,and examples of the olefin include ethylene, propylene, butene, pentene,hexene, heptene, and octene. The olefin is more preferably ethylene.Examples of the α,β-unsaturated carboxylic acid having 3 to 8 carbonatoms include acrylic acid, methacrylic acid, fumaric acid, maleic acid,and crotonic acid. Among these, acrylic acid and methacrylic acid areparticularly preferred. Examples of the α,β-unsaturated carboxylic acidester include methyl ester, ethyl ester, propyl ester, n-butyl ester,isobutyl ester of acrylic acid, methacrylic acid, fumaric acid, andmaleic acid. In particular, acrylic acid ester and methacrylic acidester are preferred. Among these, the ionomer resin is preferably ametal ion-neutralized product of a binary copolymer composed of ethyleneand (meth)acrylic acid, and a metal ion-neutralized product of a ternarycopolymer composed of ethylene, (meth)acrylic acid and (meth)acrylicacid ester.

The cover composition constituting the cover of the golf ball accordingto the present invention preferably contains, as a resin component, athermoplastic polyurethane elastomer or an ionomer resin. In case ofusing the ionomer resin, it is preferred to use a thermoplastic styreneelastomer in combination. The amount of the polyurethane or ionomerresin in the resin component of the cover composition is preferably 50mass % or more, more preferably 60 mass % or more, and even morepreferably 70 mass % or more.

In addition to the above resin component, the cover composition mayfurther contain a pigment component such as a white pigment (e.g.titanium oxide), a blue pigment and a red pigment; a weight adjustingagent such as zinc oxide, calcium carbonate and barium sulfate; adispersant; an antioxidant; an ultraviolet absorber; a light stabilizer;a fluorescent material or a fluorescent brightener; and the like, aslong as they do not impair the performance of the cover.

The amount of the white pigment (e.g. titanium oxide) is preferably 0.5part by mass or more, more preferably 1 part by mass or more, and ispreferably 10 parts by mass or less, more preferably 8 parts by mass orless, with respect to 100 parts by mass of the resin componentconstituting the cover. This is because if the amount of the whitepigment is 0.5 part by mass or more, it is possible to impart theopacity to the cover, and if the amount of the white pigment is morethan 10 parts by mass, the durability of the obtained cover maydeteriorate.

It is preferred that the slab hardness of the cover composition isappropriately set in accordance with the desired performance of the golfball. For example, in case of a so-called distance golf ball whichfocuses on a flight distance, the cover composition preferably has aslab hardness of 50 or more, more preferably 55 or more, and even morepreferably 60 or more, and preferably has a slab hardness of 80 or less,more preferably 70 or less, and even more preferably 68 or less in ShoreD hardness. If the cover composition has a slab hardness of 50 or more,the obtained golf ball has a higher launch angle and lower spin rate ondriver shots and iron shots, and thus travels a greater flight distance.Further, if the cover composition has a slab hardness of 80 or less, thegolf ball excellent in durability is obtained. In addition, in case of aso-called spin golf ball which focuses on controllability, the covercomposition preferably has a slab hardness of less than 50, andpreferably has a slab hardness of 20 or more, more preferably 25 ormore, and even more preferably 30 or more in Shore D hardness. If thecover composition has a slab hardness of less than 50 in Shore Dhardness, the obtained golf ball readily stops on the green due to thehigh spin rate on approach shots. In addition, if the cover compositionhas a slab hardness of 20 or more in Shore D hardness, the abrasionresistance becomes better. In case of a plurality of cover layers, theslab hardness of the cover composition constituting each layer may beidentical to or different from each other.

Examples of the method for molding the cover of the golf ball of thepresent invention include a method which comprises molding the covercomposition into a hollow-shell, covering the core with a plurality ofthe hollow-shells and subjecting the core with a plurality of the hollowshells to the compression molding (preferably a method which comprisesmolding the cover composition into a half hollow-shell, covering thecore with two of the half hollow-shells, and subjecting the core withtwo of the half hollow-shells to the compression molding); and a methodwhich comprises injection molding the cover composition directly ontothe core.

When molding the cover in the compression molding method, molding of thehalf shell may be conducted by either a compression molding method or aninjection molding method, but the compression molding method ispreferred. The compression molding of the cover composition into thehalf shell can be carried out, for example, under a pressure of 1 MPa ormore and 20 MPa or less at a temperature of −20° C. or more and 70° C.or less relative to the flow beginning temperature of the covercomposition. By performing the molding under the above conditions, ahalf shell having a uniform thickness can be formed. Examples of themethod for molding the cover using half shells include a method ofcovering the core with two of the half shells and then subjecting thecore with two of the half shells to the compression molding. Thecompression molding of the half shells into the cover can be carriedout, for example, under a pressure of 0.5 MPa or more and 25 MPa or lessat a temperature of −20° C. or more and 70° C. or less relative to theflow beginning temperature of the cover composition. By performing themolding under the above conditions, a golf ball cover having a uniformthickness can be formed.

In case of injection molding the cover composition into the cover, thecover composition extruded in a pellet form beforehand may be used forthe injection molding, or the cover materials such as the base resincomponents and the pigment may be dry blended, followed by directlyinjection molding the blended material. It is preferred to use upper andlower molds having a hemispherical cavity and pimples for forming thecover, wherein a part of the pimples also serves as a retractable holdpin. When molding the cover by the injection molding, the hold pin isprotruded to hold the core, and the cover composition is charged andthen cooled to obtain the cover. For example, the molding of the covermay be conducted as follows: the cover composition heated to atemperature ranging from 200° C. to 250° C. is charged for 0.5 to 5seconds into a mold held under a pressure of 9 MPa to 15 MPa, and aftercooling for 10 to 60 seconds, the mold is opened.

When molding the cover, concave portions called “dimple” are usuallyformed on the surface of the cover. The total number of dimples formedon the cover is preferably 200 or more and 500 or less. If the totalnumber is less than 200, the dimple effect is hardly obtained. On theother hand, if the total number exceeds 500, the dimple effect is hardlyobtained because the size of the respective dimples is small. The shape(shape in a plan view) of dimples includes, for example, withoutlimitation, a circle, a polygonal shape such as a roughly triangularshape, a roughly quadrangular shape, a roughly pentagonal shape, aroughly hexagonal shape, and other irregular shape. The shape of dimplesis employed solely or at least two of them may be used in combination.

The thickness of the cover is preferably 4.0 mm or less, more preferably3.0 mm or less, and even more preferably 2.0 mm or less. If thethickness of the cover is 4.0 mm or less, the resilience and shotfeeling of the obtained golf ball become better. The thickness of thecover is preferably 0.3 mm or more, more preferably 0.5 mm or more, evenmore preferably 0.8 mm or more, and most preferably 1.0 mm or more. Ifthe thickness of the cover is less than 0.3 mm, the durability and wearresistance of the cover may deteriorate. In the case that the cover hasa plurality of layers, it is preferred that the total thickness of thecover layers falls within the above range.

The golf ball body having the cover formed thereon is ejected from themold, and is preferably subjected to surface treatments such asdeburring, cleaning and sandblast where necessary. In addition, ifdesired, a paint film or a mark may be formed. The paint film preferablyhas a thickness of, but is not particularly limited to, 5 μm or more,more preferably 7 μm or more, and preferably has a thickness of 50 μm orless, more preferably 40 μm or less, even more preferably 30 μm or less.If the thickness of the paint film is less than 5 μm, the paint film iseasy to wear off due to the continued use of the golf ball, and if thethickness of the paint film exceeds 50 μm, the dimple effect is reducedand thus the flight performance of the golf ball may be lowered.

[Golf Ball]

The golf ball construction of the present invention is not particularlylimited, as long as the golf ball comprises a core and at least onecover covering the core. The FIGURE is a partially cutawaycross-sectional view of a golf ball according to one embodiment of thepresent invention. The golf ball 1 comprises a spherical core 2, and acover 3 covering the spherical core 2. A plurality of dimples 31 areformed on the surface of the cover. Other portion than the dimples 31 onthe surface of the golf ball 1 is a land 32. The golf ball 1 is providedwith a paint layer and a mark layer outside the cover 3, but theselayers are not depicted.

The core is preferably spherical. Further, the core may have either asingle layered structure or a multiple layered structure, but the singlelayered structure is preferable. This is because unlike the core havingthe multiple layered structure, the core having the single layeredstructure does not have energy loss at the interface of the multiplelayered structure when being hit, and thus has higher resilience. Inaddition, the cover has a structure composed of at least one layer, forexample, a single layered structure, or a multiple layered structurecomposed of at least two layers. Examples of the golf ball according tothe present invention include a two-piece golf ball composed of a coreand a single layered cover disposed around the core; a multi-piece golfball (including a three-piece golf ball) composed of a core and at leasttwo cover layers disposed around the core; and a wound golf ballcomposed of a core, a rubber thread layer which is formed around thecore, and a cover disposed around the rubber thread layer. The presentinvention can be suitably applied to any one of the above golf balls.

The golf ball of the present invention preferably has a diameter in arange from 40 mm to 45 mm. In light of satisfying a regulation of USGolf Association (USGA), the diameter is particularly preferably 42.67mm or more. In light of prevention of air resistance, the diameter ismore preferably 44 mm or less, particularly preferably 42.80 mm or less.In addition, the golf ball of the present invention preferably has amass of 40 g or more and 50 g or less. In light of obtaining greaterinertia, the mass is more preferably 44 g or more, particularlypreferably 45.00 g or more. In light of satisfying a regulation of USGA,the mass is particularly preferably 45.93 g or less.

When the golf ball of the present invention has a diameter in a rangefrom 40 mm to 45 mm, the compression deformation amount of the golf ball(shrinking amount of the golf ball along the compression direction) whenapplying a load from 98 N as an initial load to 1275 N as a final loadto the golf ball is preferably 2.0 mm or more, more preferably 2.3 mm ormore, and even more preferably 2.5 mm or more, and is preferably 4.0 mmor less, more preferably 3.3 mm or less, and even more preferably 3.5 mmor less. If the compression deformation amount is 2.0 mm or more, thegolf ball does not become excessively hard, and thus the shot feelingthereof becomes better. On the other hand, if the compressiondeformation amount is 4.0 mm or less, the resilience of the golf ballbecomes higher.

EXAMPLES

Next, the present invention will be described in detail by way ofexamples. However, the present invention is not limited to the examplesdescribed below. Various changes and modifications without departingfrom the spirit of the present invention are included in the scope ofthe present invention.

[Evaluation Methods] (1) Compression Deformation Amount

A compression deformation amount of the core or golf ball (a shrinkingamount of the core or golf ball along the compression direction), whenapplying a load from an initial load of 98 N to a final load of 1275 Nto the core or golf ball, was measured.

(2) Core Hardness (Shore C Hardness)

The Shore C hardness measured with an automatic hardness tester DigitestII available from Bareiss company at the surface of the core was adoptedas the surface hardness of the core. In addition, the core was cut intotwo hemispheres to obtain a cut plane, and the hardness at the centralpoint of the cut plane and the hardness at the point located at thedistance of 75% of the core radius from the central point of the cutplane were measured. It is noted that the hardness at the point locatedat the distance of 75% of the core radius from the central point of thecut plane was calculated by measuring the hardness values at four pointslocated at the distance of 75% of the core radius from the central pointof the cut plane and averaging the hardness values measured at the fourpoints.

(3) Slab Hardness (Shore D Hardness)

Sheets with a thickness of about 2 mm were produced by injection moldingthe cover composition. The sheets were stored at 23° C. for two weeks.At least three of these sheets were stacked on one another so as not tobe affected by the measuring substrate on which the sheets were placed,and the hardness of the stack was measured with an automatic hardnesstester (Digitest II, available from Bareiss company) using a testingdevice of “Shore D”.

(4) Shot Feeling

An actual hitting test was carried out by ten amateur golfers (highskilled persons) using a driver. In accordance with the followinggrading standard, the feeling of each golfer at hitting the golf ballwas evaluated. The shot feeling most evaluated by the ten golfers wasadopted as the shot feeling of that golf ball.

Grading Standard

E (excellent): Impact is small and feeling is good.

G (good): There is impact but feeling is good one.

F (Fair): Normal

P (Poor): Impact is great and feeling is bad.

[Production of Golf Ball] (1) Production of Core

The rubber compositions having the formulations shown in Tables 1 to 3were kneaded with a kneading roll, and heat pressed in upper and lowermolds, each having a hemispherical cavity, at 170° C. for 30 minutes toobtain spherical cores having a diameter of 40.0 mm. It is noted thatbarium sulfate was added in an appropriate amount such that the obtainedgolf balls had a mass of 45.3 g.

TABLE 1 Golf ball No. 1 2 3 4 5 6 7 Standard Core rubber Formulation (a)BR730 100 100 100 100 100 100 100 100 composition (parts by mass) (b)ZN-DA90S 32 34 36 40 44 48 52 29 (c) Dicumyl peroxide 0.8 0.8 0.8 0.80.8 0.8 0.8 0.8 (d) Sylvares TP2019 1 3 5 10 15 20 25 0 (e) PBDS 0.3 0.30.3 0.3 0.3 0.3 0.3 0.3 (f) Zinc oxide 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0Barium sulfate * * * * * * * * Blending ratio (b)/(d) of component (b)32.0 11.3 7.2 4.0 2.9 2.4 2.1 — to component (d) Softening point ofcomponent (d) (° C.) 125 125 125 125 125 125 125 — Core Compressiondeformation amount (mm) 3.3 3.3 3.3 3.3 3.3 3.3 3.3 3.3 Center hardnessHo (Shore C) 57.5 60.0 62.5 60.7 66.0 55.0 55.0 57.5 Hardness H75 at thedistance of 75% 69.9 70.0 70.5 69.7 69.4 70.0 60.0 69.9 from center(Shore C) Surface hardness Hs (Shore C) 83.1 79.1 72.3 72.8 72.2 70.163.1 83.1 Hs − H75 13.2 9.1 1.8 3.1 2.8 0.1 3.1 13.2 Hs − Ho 25.6 19.19.8 12.1 6.2 15.1 8.1 25.6 Golf Compression deformation amount (mm) 3.03.0 3.0 3.0 3.0 3.0 3.0 3.0 ball Shot feeling on driver shots F G E E EG F P * Appropriate amount

TABLE 2 Golf ball No. 8 9 10 11 12 13 14 Standard Rubber Formulation (a)BR730 100 100 100 100 100 100 100 100 composition (parts by mass) (b)ZN-DA90S 32 34 36 40 44 48 52 29 (g) Dicumyl peroxide 0.8 0.8 0.8 0.80.8 0.8 0.8 0.8 (d) YS RESIN PX1150N 1 3 5 10 15 20 25 0 (e) PBDS 0.30.3 0.3 0.3 0.3 0.3 0.3 0.3 (f) Zinc oxide 5.0 5.0 5.0 5.0 5.0 5.0 5.05.0 Barium sulfate * * * * * * * * Blending ratio (b)/(d) of component(b) 32.0 11.3 7.2 4.0 2.9 2.4 2.1 — to component (d) Softening point ofcomponent (d) (° C.) 115.0 115.0 115.0 115.0 115.0 115.0 115.0 — CoreCompression deformation amount (mm) 3.3 3.3 3.3 3.3 3.3 3.3 3.3 3.3Center hardness Ho (Shore C) 57.5 58.0 58.8 59.6 53.8 55.0 55.0 57.5Hardness H75 at the distance of 75% 69.9 70.0 71.8 70.8 67.4 70.0 60.069.9 from center (Shore C) Surface hardness Hs (Shore C) 83.1 79.1 78.575.5 72.6 70.1 63.1 83.1 Hs − H75 13.2 9.1 6.7 4.7 5.2 0.1 3.1 13.2 Hs −Ho 25.6 21.1 19.7 15.9 18.8 15.1 8.1 25.6 Golf Compression deformationamount (mm) 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 ball Shot feeling on drivershots F G E E E G F P * Appropriate amount

TABLE 3 Golf ball No. 15 16 17 18 19 20 21 Standard Rubber Formulation(a) BR730 100 100 100 100 100 100 100 100 composition (parts by mass)(b) ZN-DA90S 29 31 33 37 41 45 49 29 (c) Dicumyl peroxide 0.8 0.8 0.80.8 0.8 0.8 0.8 0.8 (d) Sylvatraxx 6720 1 3 5 10 15 20 25 0 (e) PBDS 0.30.3 0.3 0.3 0.3 0.3 0.3 0.3 (f) Zinc oxide 5.0 5.0 5.0 5.0 5.0 5.0 5.05.0 Barium sulfate * * * * * * * * Blending ratio (b)/(d) of component(b) 29.0 10.3 6.6 3.7 2.7 2.3 2.0 — to component (d) Softening point ofcomponent (d) (° C.) 118 118 118 118 118 118 118 — Core Compressiondeformation amount (mm) 3.3 3.3 3.3 3.3 3.3 3.3 3.3 3.3 Center hardnessHo (Shore C) 57.5 60.0 62.5 60.7 66.0 55.0 55.0 57.5 Hardness H75 at thedistance of 75% 69.9 70.0 70.5 69.7 69.4 70.0 60.0 69.9 from center(Shore C) Surface hardness Hs (Shore C) 83.1 79.1 72.3 72.8 72.2 70.163.1 83.1 Hs − H75 13.2 9.1 1.8 3.1 2.8 0.1 3.1 13.2 Hs − Ho 25.6 19.19.8 12.1 6.2 15.1 8.1 25.6 Golf Compression deformation amount (mm) 3.03.0 3.0 3.0 3.0 3.0 3.0 3.0 ball Shot feeling on driver shots F G E E EG F P * Appropriate amount

The materials used in Tables 1 to 3 are shown below.

BR730: high-cis polybutadiene rubber (amount of cis-1,4 bond=95 mass %,amount of 1,2-vinyl bond=1.3 mass %, Moony viscosity (ML₁₊₄ (100°C.)=55, molecular weight distribution (Mw/Mn)=3) available from JSRCorporation

ZN-DA90S: zinc acrylate available from Nisshoku Techno Fine ChemicalCo., Ltd.

Dicumyl peroxide: available from Tokyo Chemical Industry Co., Ltd.

Sylvares TP2019 (pinene-phenol copolymer, softening point: 125° C.):available from KRATON CORPORATION

YS RESIN PX1150N (β-pinene polymer, softening point: 115±5° C.):available from Yasuhara Chemical Co. Ltd.

Sylvatraxx 6720 (pinene-α-methylstyrene-phenol copolymer, softeningpoint: 118° C.): available from KRATON CORPORATION

PBDS: bis(pentabromophenyl) disulfide available from Kawaguchi ChemicalIndustry Co., Ltd.

Zinc oxide: “Ginrei R” available from Toho Zinc Co., Ltd.

Barium sulfate: “Barium Sulfate BD” available from Sakai ChemicalIndustry Co., Ltd.

(2) Production of Cover and Production of Golf Ball

Cover materials having the formulation shown in Table 4 were extrudedwith a twin-screw kneading extruder to prepare a cover composition in apellet form. The extruding conditions of the cover composition were ascrew diameter of 45 mm, a screw rotational speed of 200 rpm, and screwL/D=35, and the mixture was heated to 160 to 230° C. at the die positionof the extruder. The obtained cover composition was injection moldedonto the spherical core obtained above to produce a golf ball comprisinga spherical core and a cover covering the spherical core. Evaluationresults of the obtained golf balls are shown in Tables 1 to 3.

TABLE 4 Cover composition No. 1 Himilan 1555 47 Himilan 1557 46 TEFABLOCT3221C 7 Titanium dioxide (A220) 4 JF-90 0.2 Hardness (Shore D) 57Formulation: parts by mass

The materials used in Table 4 are shown below.

Himilan 1555: Na-neutralized ionomer available from Du Pont-MitsuiPolychemicals Co., Ltd.

Himilan 1557: Zn-neutralized ionomer available from Du Pont-MitsuiPolychemicals Co., Ltd.

TEFABLOC T3221C: thermoplastic elastomer containing styrene blockavailable from Mitsubishi Chemical Corporation

A220: titanium dioxide available from Ishihara Sangyo Kaisha, Ltd.

JF-90: light stabilizer available from Johoku chemical Co., Ltd.

As shown in Tables 1 to 3, the golf balls according to the presentinvention comprising a core and at least one cover covering the core,wherein the core is formed from a core rubber composition containing (a)a base rubber containing a polybutadiene, (b) an α,β-unsaturatedcarboxylic acid having 3 to 8 carbon atoms and/or a metal salt thereofas a co-crosslinking agent, (c) a crosslinking initiator, and (d) aterpene-based resin, each have excellent shot feeling on driver shots.

The golf ball according to the present invention has excellent shotfeeling on driver shots. This application is based on Japanese patentapplication No. 2018-185319 filed on Sep. 28, 2018, the content of whichis hereby incorporated by reference.

1. A golf ball comprising a core and at least one cover covering thecore, wherein the core is formed from a core rubber compositioncontaining (a) a base rubber containing a polybutadiene, (b) anα,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or ametal salt thereof as a co-crosslinking agent, (c) a crosslinkinginitiator, and (d) a terpene-based resin.
 2. The golf ball according toclaim 1, wherein (d) the terpene-based resin is at least one memberselected from the group consisting of a terpene polymer, aterpene-phenol copolymer, a terpene-styrene copolymer, aterpene-phenol-styrene copolymer, a hydrogenated terpene-phenolcopolymer, a hydrogenated terpene-styrene copolymer, and a hydrogenatedterpene-phenol-styrene copolymer.
 3. The golf ball according to claim 1,wherein the terpene-based resin is at least one member selected from thegroup consisting of compounds having a structure represented by thefollowing formulae (1) to (4):

wherein in the formulae (1) to (4), R¹ and R² each independentlyrepresent a divalent residue of a phenol-based compound and/or astyrene-based compound, m¹ to m⁴ each independently represent a naturalnumber of 1 to 30, and n¹ to n² each independently represent a naturalnumber of 1 to
 20. 4. The golf ball according to claim 1, wherein (d)the terpene-based resin is at least one member selected from the groupconsisting of an α-pinene-phenol copolymer, an α-pinene-α-methylstyrenecopolymer, an α-pinene-α-methylstyrene-phenol copolymer, a3-pinene-phenol copolymer, a β-pinene-α-methylstyrene copolymer, and aβ-pinene-α-methylstyrene-phenol copolymer.
 5. The golf ball according toclaim 1, wherein (d) the terpene-based resin has a softening point in arange of from 60° C. to 150° C.
 6. The golf ball according to claim 1,wherein the core rubber composition contains (d) the terpene-based resinin an amount of from 3 parts by mass to 20 parts by mass with respect to100 parts by mass of (a) the base rubber.
 7. The golf ball according toclaim 1, wherein a blending ratio (component (b)/component (d)) of thecomponent (b) to the component (d) ranges from 2.0 to 15.0 in a massratio.
 8. The golf ball according to claim 1, wherein the polybutadieneis a high-cis polybutadiene having a cis-1,4 bond in an amount of 90mass % or more.
 9. The golf ball according to claim 1, wherein ahardness (H75) at a point located at a distance of 75% of a core radiusfrom a center of the core ranges from 55 to 80 in Shore C hardness. 10.The golf ball according to claim 1, wherein a surface hardness (Hs) ofthe core ranges from 60 to 83 in Shore C hardness.
 11. The golf ballaccording to claim 1, wherein a central hardness (Ho) of the core rangesfrom 30 to 70 in Shore C hardness.
 12. The golf ball according to claim1, wherein a hardness difference (Hs−Ho) between a surface hardness (Hs)of the core and a central hardness (Ho) of the core ranges from 5 to 35in Shore C hardness.
 13. The golf ball according to claim 1, wherein ahardness difference (H75−Ho) between a hardness (H75) at a point locatedat a distance of 75% of a core radius from a center of the core and acentral hardness (Ho) of the core ranges from 3 to 20 in Shore Chardness.
 14. The golf ball according to claim 1, wherein a hardnessdifference (Hs−H75) between a hardness (H75) at a point located at adistance of 75% of a core radius from a center of the core and a surfacehardness (Hs) of the core is less than 10 in Shore C hardness.
 15. Thegolf ball according to claim 2, wherein the terpene-phenol copolymer hasan acid value in a range of from 10 mgKOH/g 300 mgKOH/g.
 16. The golfball according to claim 2, wherein the terpene-phenol copolymer has ahydroxy value in a range of from 30 mgKOH/g to 150 mgKOH/g.
 17. A golfball comprising a core and at least one cover covering the core, whereinthe core is formed from a core rubber composition containing (a) a baserubber containing a polybutadiene, (b) an α,β-unsaturated carboxylicacid having 3 to 8 carbon atoms and/or a metal salt thereof as aco-crosslinking agent, (c) a crosslinking initiator, and (d) aterpene-based resin, the core rubber composition contains (d) theterpene-based resin in an amount of from 3 parts by mass to 20 parts bymass with respect to 100 parts by mass of (a) the base rubber, ahardness difference (Hs−H75) between a hardness (H75) at a point locatedat a distance of 75% of a core radius from a center of the core and asurface hardness (Hs) of the core is less than 10 in Shore C hardness,and the terpene-based resin is at least one member selected from thegroup consisting of compounds having a structure represented by thefollowing formulae (1) to (4):

wherein in the formulae (1) to (4), R¹ and R² each independentlyrepresent a divalent residue of a phenol-based compound and/or astyrene-based compound, m¹ to m⁴ each independently represent a naturalnumber of 1 to 30, and n¹ to n² each independently represent a naturalnumber of 1 to 20.